The present invention relates to a control method and system for an high voltage direct current (hereinafter referred to as "HVDC") system.
In a large HVDC system, a converter station comprises converters connected in cascade. Under a certain operation mode where only one or two of the converters operate and the remaining converters are bypassed by a corresponding suitable switch, it is necessary for another converter(s) out of operation to operate in addition to the operating converter(s) in accordance with some request, for example, a demand for an increase in transmitted power in the HVDC system. On the contrary, under another operation mode where all the converters operate normally, it is necessary for one or two of the converters to be bypassed by a corresponding suitable switch in accordance with some requests, for example, a demand for a decrease in transmitted power in the HVDC system.
Under another mode, for example, when a HVDC system is operating normally, it is necessary for the direction of power transmission in the HVDC system to be changed in accordance with a command given by a central control station.
When the above modes occurred, the values of the control angles given to the converters which are to take part in operation of the HVDC system or are to be changed from a rectifier operation to an inverter operation or vice versa, are changed to the suitable value of the control angles through 90.degree.. Ordinarily when a converter is in operation at the control angle value near 90.degree., the converter requires very large reactive power as is well known. In case that the converter is connected to a weak a.c. system, therefore, the a.c. voltage of the converter decreases. This causes unstableness or inoperativeness of the converter operation of the HVDC system.
To prevent the unstableness or inoperativeness of the converter operation, therefore, it is necessary for the converter station of the HVDC system connected to the weak A.C. system to be provided with an excessive compensation apparatus which supplies reactive power to the converter. For example, by the conventional control method or system, the capacity of the compensation apparatus is determined as follows.
CASE 1
When both the A.C. systems connected to the HVDC system are large in capacity compared with the HVDC system, the capacity of the compensation apparatus is about 60% of the capacity of HVDC system ordinarily.
CASE 2
When one of the A.C. systems connected to the HVDC system is weak compared with the capacity of the HVDC system, the capacity of the compensation apparatus is about 112% of the capacity of the HVDC system.
Because, in case 2, the reactive power which is necessary to operate the converter is not supplied fully from the weak A.C system, the capacity of the compensation apparatus in case 2 is much larger than that in case 1.
This means that the conventional method and system require large initial cost to construct the HVDC system. Further, under normal operation, since the reactive power required is smaller than the capacity of the compensation apparatus, a utilization factor of the compensation apparatus is small.